lvlax
you've had many suggestions per caliber/cartridge. Find and know the legal minimum of your intended area.
You titled this thread, " If YOU can have only one b a rifle..."
I have several diff brands and types and have had even more and it's a hard decision for me to pick ONE,
PROBABLY if I had to be limited to ONE it would be....
EITHER one of these. Right -- T 3 Lite SS 270 Win > > > Left -- T3X Lite SS 7mm RM
Considering hunting a variety of places & game, I'd probably pick the 7 Rem Mag.
I don't think that would be best for you & your situation. But I like it.
Jerry